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Technical question re: ROP/LOP


RobertE

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Just a technical question, folks.  No way am I opening the whole topic to debate.

My question concerns optimal timing for LOP operations.  I think it's established fact that the flame front is slower LOP.  So, really, two questions.  First, does anyone know how much slower? 2 percent slower? 5 percent? 10 percent? Second, if, say, the flame front is 10 percent slower and since the engine manufacturers assume ROP operation and establish the timing accordingly  then does It follow that the optimal timing for LOP should be adjusted proportionately?  For instance, if optimal timing ROP is 20 degrees then is optimal timing LOP 20/.9, or 22.2 degrees BTDC?

Thanks.

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Technical thoughts... :) (where is Ross?  Our go to guy for flame front discussions...)

1) There are geometry limits to setting the timing too far in advance...

2) The idea is to get the flame propagating so it is wide spread just after TDC...and completed as much as possible before the exhaust valve opens...

3) Setting the ignition too far in advance wil have exhaust gasses expanding too much before TDC, while the charge is still compressing... resulting in high heat and other problems.  Engine ping?

4) Slowing the RPM down also increases the amount time for the flame front to spread.

5) The math on This has got to be tremendous for simulation.  Changing volumes, changing temperatures, pressures, and changing rates of reactions...

6) Real life data, and a group of knowledgeable people, sharing real life experience is incredibly helpful, in lieu of a supercomputer and a genius programmer...

7) Our real life data gathering has two options for the IO360.  25 and 20° BTDC... both work.  One converts more fuel to power before the exhaust valve opens... the other keeps the cylinders a bit cooler...

8) what happens if we go 26 and 19° BTDC... hotter CHTs and smaller degree of safety from engine ping, vs even cooler CHTs and less fuel converted to power...

9) The Continental engine use a compromise timing of 22 or 23° BTDC...

10) Lots of engine safety is considered while selecting the timing.  If the plane is run with high OATs all the time, advanced timing may not be that good...

11) So all that out in the open...  LOP allows for higher conversion of fuel to power, ONLY because less excess fuel goes out the exhaust valve.  This is a chemistry / Stoichiometry question not a timing question...

12) I believe, using timing to initiate the flame front early, is valid for both ROP and LOP... the limitations are again expanding exhaust prior to TDC and time before the valve opens...  

13) flame spread is slower in either direction of PEAK... excess molecules of either 100LL or O2 get in the way...  more LOP or More ROP is going to slow things down... based on the statistics of collisions between proper molecules...

14) ever see an engine run ROP with short exhaust stacks? The flames are visible coming out the pipes. A real obvious sign that fuel is being converted outside the cylinders... (not the same conditions in actual flight...)

15) As Byron points out... in cruise a different timing would be preffered over full power T/O and climb timing... as is the RPM of choice.  

16) If ignition timing could be changed during the flight... That would be nice...  making sure the timing is reset before a go around... would be critical...  melted pistons can happen in a few minutes under tough conditions.  In a Go-around situation, losing a piston would be terrible...

Now, who has the data or the math to share? :)

Best regards,

-a-

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I came across other forms of timing used in engines... supporting Byron's claim that we can't get enough advance for LOP in cruise.

This paragraph discusses using mechanical methods to increase the timing BTDC.  Modern cars run LOP with similar fuels.... they are 30-40° BTDC according to this article...  wide open throttle minimizes the advance... closing down the throttle(low MP) increases the advance...

 

Mechanical advance is the second method of ignition timing advance. As the distributor spins fast enough to activate the mechanical-advance weights, the engine receives initial timing, mechanical timing, and vacuum timing under part-throttle conditions. As the engine accelerates to wide-open throttle (WOT), the vacuum drops, eliminating the vacuum canister’s timing. For example, part-throttle total timing would look something like this: 10 degrees initial + 10 degrees vacuum + 20 degrees mechanical = 40 degrees of total timing. At WOT, there is no vacuum present and the canister timing is eliminated, giving your engine a total of 30 degrees timing. The reason your engine is able to sustain more timing at part-throttle is because only a limited amount of air and fuel make it into the cylinder at part-throttle. Lower cylinder pressures enable the combustion process to start sooner and help improve part-throttle response by increasing torque. This additional part-throttle timing improves efficiency and torque.

 

If we go electronic ignition, with an MP sensor connected to the timing, can we adjust the timing back towards 30°BTDC...

Would that be possible with a proper STC?

Best regards,

-a-

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It ultimately boils down to detonation margins. The engine is set up so it wont detonate at the highest power setting. We have fixed timing so it can't be changed in flight. 

Would our engines run more efficiently with more advance at partial power? Yes, but unlike a car we operate in a very narrow range of MP and RPM so there is little advantage to variable timing. 

If we had a timing knob, we could get a few percent more power at cruise, but if you forgot to reset it at the next takofff you could destroy your engine. Is it worth the risk?

Edited by N201MKTurbo
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There is a company that produces a complete electronic fuel injection and and ignition system that could be put on our engines if we were not in the certified category.  I like the system and would love to install it on my plane.

If I am not mistaken don't some of the electronic ignition systems that have been approved for our planes vary the timing?  Do they go above 25 degree BTDC?

At altitude the lower MP in effect simulates a partially closed throttle position does it not?  So we cold benefit form variable timing in cruise flight.

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At altitude, I see about 20" MP @ WOT.  I can simulate that at lower altitudes by using partial MP...

1) Picking up a few percent of efficiency would be measured in gallons @$4 each...

2) adjusting Timing for the start process could really improve the hot start anxiety...

3) deciding where to put the EFI decal would be the hard part... somewhere near the 9.0 badge, Mustang like...

I sense electronic ignition is coming... and not that far away...

Best regards,

-a-

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